1. Field of the Invention
This invention relates to apparatus for producing displays of data signals, for example, analog physiological signals, and relates specifically to so-called X-Y or vector cathode ray tube (CRT) display apparatus.
2. Description of the Prior Art
Display apparatus utilizing cathode ray tubes for providing single or multi-channel displays and/or records of analog or digital information are known in the prior art. The cathode ray tube embodied in such apparatus includes a transparent display face or screen on the inside of which is a phosphor coating. The electron beam is impinged on and is swept horizontally and vertically across the phosphor coating. This produces a moving spot of light which is visible from the front of the face of the tube. In one form of such apparatus curves representative of the information under measurement, study and/or analysis are plotted as continuous lines or traces. In other forms of such apparatus, such curves are plotted as a series of separated spots or dots.
In a cathode ray tube the emitted light intensity is proportional to the energy of the electron beam which impinges on the phosphor coating. In practice the potentials of the control elements are adjusted to such a value that the beam does not unduly heat or burn or otherwise injure the phosphor coating at any spot where the beam may remain at rest for any appreciable period. Where increased light intensity is desired, however, an increase in the beam current is required. This produces greater heating of the phosphor coating in the area of beam impingement, particularly when the beam is repetitively applied to a single position or is slowly moving, with a resultant shortening of the useful life of the tube. Repetitive scanning along the same path by the cathode ray tube beam tends, also, to result in eventual burning of the phosphor coating. This, in turn, causes a reduction in the effective light intensity. Thus, a problem inherent in prior art cathode ray tube display devices is the relatively short life of the phosphor coating particularly in applications where the information being displayed is slowly changing or is constantly slowly retracing the same path.
It has been proposed in the prior art to prolong the useful life of the phosphor coating in cathode ray tube display apparatus by spreading out the area of the beam impingement. This is effected by focusing and defocusing the beam on the phosphor coating at a predetermined modulation frequency. Such an arrangement is shown in U.S. Pat. No. 3,441,668, S. E. Townsend. It has also been proposed in the prior art to shift the path of the beam transversely to the direction of repetitive sweep, at a suitable rate, as shown in U.S. Pat. No. 3,700,955, Peter R. Lowe.
A disadvantage of the prior art apparatus first mentioned above, however, is a loss in sharpness of the trace produced. Additionally, when the information being displayed is not changing or only changing slowly, the continual or repetitive impingement of the beam on the same spot tends to cause phosphor fatigue or a burn in the phosphor coating, notwithstanding the provisions made for modulating the beam focusing to avoid such result. A disadvantage of the second mentioned prior art apparatus is the distortion produced in the curve as a result of the transverse shifting of the beam.
Two general types of cathode ray tube display apparatus are known in the art. One is a so-called raster scan or repetitive line scan display. The other is an X-Y or vector display. The prior art solutions for prolonging the life of the cathode ray tube phosphor coating have been in connection with raster scan or line scan dislay apparatus and are not practically suitable for X-Y or vector scan display apparatus aside from the inherent disadvantage of those solutions.